Reconstruction of the 01 February 1814 eruption of Mayon Volcano, Philippines

Mayan Volcano's eruption on 01 February 1814 is considered as the volcano's most violent eruption episode, devastating five towns in the southern slopes of the volcano and killing at least 1,200 people. The deposits of the 1814 eruption are mainly distributed on the southern slopes of the volcano. The primary volcanic succession consists of, from bottom to top, tephra fall deposit, lower ignimbrite, pyroclastic surge deposit and upper ignimbrite. Two post-eruption lahar units were also recognized in the field area. The tephra fall unit, although not observed in direct contact with any of the other primary deposits, was distinguished based on petrologic and geochemical similarities with the lower ignimbrite and pyroclastic surge deposit. The lower ignimbrite and the overlying pyroclastic surge deposit are both scoriaceous, and are similarly bombs-rich; the surge deposit is distinguished by its characteristically good sorting. In contrast, the upper ignimbrite contains abundant angular altered clasts derived from pre-eruption deposits. All the primary deposits are interpreted to have been derived from an eruption column that was generated by multiple explosive eruptions occurring in close succession. This column initially generated the tephra fall. Discrete phases of column collapse produced the succession of lower ignimbrite, pyroclastic surge deposit and upper ignimbrite. The wide dispersal, composition and textural characteristics of the pyroclastic surge indicate that it was generated by a discrete phase of an eruption column collapse. The upper ignimbrite is the deposit from a density current produced during the cessation of the eruption that was accompanied by partial collapse of the crater wall. The 1814 deposits are predominantly composed of basaltic andesite, with minor more acidic andesite. Petrographic texture and contact relationships, bimodal distribution of plagioclase, and variation in glass composition indicate mixing of two magmas. A geologic model for the 1814 eruption is proposed in which an intermediate andesite magma residing in a small, shallow chamber beneath Mayan was intruded by a comparably larger magma of basaltic andesite composition. The resulting magma mixing may have triggered the explosive eruption of 1814

トカラ レットウ ニ オケル チュウキ コウシンセイ ノ サンセイ カイテイ カザン カツドウ

To understand the submarine volcanism surrounding the Tokara Islands, a submarine topographic analysis and 67 dredge samplings were carried out. Prior to the submarine investigations, we reviewed comprehensively geological and geophysical data on this region and confirmed the complexity of both volcanic activity and tectonic setting of the Tokara Islands. In contrast to the homogeneous subaerial volcanic rocks comprising predominantly two-pyroxene andesite lava flows, the dredged samples vary from basaltic andesite to rhyolite in composition. Furthermore, we reveal that dacitic and rhyolitic pumices are abundant and broadly distributed throughout the submarine area. The recovered volcanic rocks were mainly subangular to angular cobble-boulder fragments of lava, scoria, and variably vesiculated pumice. Volcanic rocks with hornblende phenocrysts occur only north of the Tokara strike-slip fault, which is a major tectonic element of volcanism. The pumices can be classified into three categories based on the size and abundance of the phenocrysts: aphyric pumice, fine-grained porphyritic pumice, and coarse-grained porphyritic pumice. Occurrences, such as amount in a dredge, shape without extensive abrasion, large fragment size, and bulk rock chemical compositions of the major pumice fragments suggest that they are in situ, rather than originating as drifted pumice or air fall, exotic pyroclastic fragments derived from the four super-eruptions of Kyushu Island. Because dredged samples contained fresh volcanic glass in the groundmass, and are not covered by iron-manganese oxide crust, they appear to have originated from the Quaternary eruptions. Indeed volcanic islands have developed above the submarine erosional terraces (indicated as knick points at approximately 110 m in depth), which is assumed to have formed during the last glacial age. K-Ar age dating on the representative pumice samples resulted in ages of 0.60 ± 0.20 Ma and < 0.2 Ma, respectively. These newly obtained submarine data support that acidic volcanisms occurred around the submarine calderas during the Mid-Pleistocene age